The present invention relates to a method for controlling a tool during machining a work piece by means of the tool.
The invention is applicable to different combinations of cutting tools and work pieces. For example, during a turning or milling operation, the tool can be controlled so as to obtain the requisite dimensions of the machined work piece. Particularly, the invention can be used during milling narrow grooves in a thin structure, such as a sheet, where the resulted wall thickness of the thin structure is critical to the performance of the final product. Such sheets are frequently used for production of combustion chambers, such as rocket nozzles which are provided with long narrow cooling channels.
In the manufacturing industry where metal work pieces are subjected to different machining operations, such as milling and turning, there are often tolerance requirements on the component resulted from the work piece, which requirements are based on for example stipulated maximum and minimum values for the dimensions of the final product.
After the machining operation, some kind of verification process has to be accomplished to secure the tolerance requirements on the component being fulfilled. Normally, the verification process is performed on the component after the machining operation has been finished, and thus the verification process and the machining process are two separate processes. This means that if the machining operation has not been successful resulting in a defective component, the component may have to be machined again or even be rejected.
For example the milling operation using a side-milling cutter to form very long narrow grooves in sheets intended for combustion chambers is a very time consuming and complex process resulting in expensive components. Particularly, conical components such as rocket nozzles having curved surfaces are very difficult to machine. Furthermore, there are very rigorous tolerance requirements on such a component which makes greats demands on accuracy of the milling process and equipments, such as tool, fixture etc., and on the verification process as well. In addition, it is not possible for practical reasons to check the wall thickness resulted from the milling operation in every single point along the bottom of the milled grooves. Another problem arises from the fact that these particular types of component often have machined grooves with a cross section size down to approximately 1×2 mm, which implies the instruments for measuring the thickness of the material must have very small dimensions to be applied in the grooves.
According to prior art ultrasonic technique is used for some types of verification of components, such as investigation of any material defects inside the material and for determination of the wall thickness of a component. The crystal of the ultrasonic probe is brought into mechanical contact with the surface of the component and the propagation of ultrasonic waves between the probe and the component is facilitated by applying a transmission medium, such as a gel on the component. Ultrasonic waves reflected from the component can give information about the interior structure of the material and the wall thickness as well. However, to achieve a useful result it is important to arrange the crystal in a certain angle relative to the surface of the component, and establish a certain contact pressure between the probe and the component. Furthermore, the gel has to be evenly distributed on the surface of the component where the probe contacts the surface of the component. Accordingly, measurements can only be performed on certain discrete points of the component and an operator has to carry out all the steps involved when moving the probe to a new measurement point over again for each point to be measured.
In another type of investigation by means of ultrasonic waves the whole component to be inspected is immersed in water used for the propagation of the ultrasonic waves. Usually, due to practical reasons this method cannot be used for large or immoveable components.
A further type of verification process is based on mechanical contact between a measuring probe, such as a metal sphere, and the component to be measured. Measurements of the variation of a magnetic field created between the probe and a second metal component arranged at the opposite side of the component give information about the wall thickness of the component.
It is desirable to provide a method of the type mentioned by way of introduction, the method enabling better control of the cutting depth or the final wall thickness, and, thus enabling a machined component to be produced with greater accuracy.
According to an aspect of the invention, a first measuring arrangement has a first ultrasonic probe for emitting ultrasonic waves, the first measuring arrangement being arranged ahead of the tool with respect to the relative tool movement direction and being moved relative to the work piece in the relative tool movement direction, wherein ultrasonic waves emitted by the first ultrasonic probe are transmitted to the work piece, and are transmitted back to the first ultrasonic probe, by means of a first column of liquid which is created by the first measuring arrangement and situated between the first ultrasonic probe and the work piece for measurement, and using the measurement for automatically adjusting the position of the tool relative to the work piece during machining the work piece.
According to an aspect of the invention, the method comprises the step of transmitting the ultrasonic waves emitted by the first ultrasonic probe to a first surface of the work piece, that the first column of liquid is situated between the first ultrasonic probe and the first surface, for measuring the distance from the first ultrasonic probe to the first surface, and using the distance measurements for automatically adjusting the position of the tool relative to the work piece during machining the work piece.
By such a method the position of the tool relative to the work piece, and, thus at least a theoretical cutting depth can continuously be calculated and the tool can be adjusted in accordance therewith. In case the conditions can be approximated to be constant, i.e. negligible tool wear etc., the calculated cutting depth is the real cutting depth.
According to a preferred embodiment of the invention, the method comprises the step of using the first ultrasonic probe for measuring the thickness of the work piece between the first surface and a second surface of the work piece, and using the thickness measurements for automatically adjusting the position of the tool relative to the work piece during machining the work piece. By such a method the process can be controlled so as to obtain a desired wall thickness of the work piece after being machined, without any prior information about the original thickness of the work piece.
According to another preferred embodiment of the invention a second measuring arrangement having a second ultrasonic probe for emitting ultrasonic waves is used, the second measuring arrangement being arranged after the tool with respect to the relative tool movement direction and being moved relative to the work piece in the relative tool movement direction, wherein ultrasonic waves emitted by the second ultrasonic probe are transmitted to a surface of the work piece which surface has been machined by the tool, and are transmitted back to the second ultrasonic probe, by means of a second column of liquid which is created by the second measuring arrangement and situated between the second ultrasonic probe and the machined surface, and the measurements accomplished by the second ultrasonic probe are used for automatically adjusting the position of the tool relative to the work piece during machining the work piece.
The second ultrasonic probe can also be used for measuring the distance between the machined surface and a second surface of the work piece, i.e. the thickness of the work piece. Alternatively, the thickness can be calculated by using the distance to the machined surface and information about the original thickness in the current position or information about the distance to the second surface obtained for example by means of the first ultrasonic probe.
By such a method the verification process can be accomplished during the machining operation and the tool can be adjusted during machining so as to compensate for tool wear and/or variation of the material characteristics or dimensions of the work piece. The verification process can be continuously accomplished and measurements can be performed at several positions along the work piece. The tool, for example a side-milling cutter blade, usually comprises one or more hard metal inserts. The performance of the tool is very much dependent on the degree of wear of these inserts. Hence it is desirable to continuously compensate for wear of the inserts so as to ensure the cutting operation will give the desired result. For this purpose the tool and the work piece can be brought towards each other based on the measurements accomplished by the ultrasonic probes.
Other advantageous features and functions of different embodiments of the invention appear from the following detailed description.